1) Field of the Invention
The present invention relates to a novel LDL receptor analog protein having a structure similar to that of LDL receptors that are responsible for the homeostasis mechanism of intracellular cholesterol and extensively participates in serum lipid metabolism, which is a critical factor that triggers the onset of arteriosclerosis. The invention also relates to the gene coding for the protein.
2) Description of the Related Art
Abnormality in serum lipid metabolism is one of the most critical risk factors in the onset and progress of arteriosclerosis. Serum lipids, together with apolipoproteins, are transformed into lipoproteins primarily in the liver, secreted therefrom, transported by blood, and taken up by a variety of tissue cells.
Uptake of lipoproteins into cells occurs primarily by the mediation of receptors of respective lipoproteins. It is known that low density lipoproteins (LDL), which are taken into cells by specific membrane receptors, called LDL receptors, are metabolized within the cells and utilized as cell membrane components or similar substances. Detailed analysis of familial hyperchlolesterolemia, which is a genetic disease accompanied by notable hyperchlolesterolemia due to abnormality of LDL receptors, has clarified details of the mechanism of homeostasis achieved by LDL receptors with respect to intracellular cholesterol.
It has been suggested that living bodies have not only LDL receptors but also cell membrane receptors that recognize other lipoproteins. From analyses of WHHL rabbits, which are model animals lacking LDL receptors, it was found that receptors which takes principally apo-E-containing lipoproteins as ligands (remnant receptors) are present in the liver. It is also predicted that there may be HDL receptors whose ligands are high density lipoprotein (HDL). However, to date, details of the structures and functions of these receptors have not yet been elucidated. It has also been known that foaming of macrophages plays an active role in the formation of atherosclerosis, is deeply participated. Macrophages foam by taking up modified LDL, not normal LDL, which have undergone oxidation, acetylation, or glycation. There have recently been discovered receptors to modified LDL which are called scavenger receptors. The scavenger receptors have been identified to be membrane receptors that have a structure completely different from that of LDL receptors.
Recent research using molecular biological techniques has identified the genes of LRP (LDL receptor-associated protein), gp 330, and VLDL receptors. The receptors have been found to have structures very similar to those of LDL receptors. From analyses of these receptors, it is believed that a plurality of lipoprotein receptors are present in living bodies, and that they are closely related to lipid metabolism. LDL receptors studied in detail by Brown and Goldstein [Brown, M. S. and Goldstein, J. L. (1986) Science 232, 34-47] are known to play an important role in the homeostasis of lipoprotein metabolism in vivo, recognizing apo-B-100 and apo-E and taking primarily LDL as their ligands. Also, LRP, which is a macroprotein, has been found to primarily recognize apo-E and to take .beta.-VLDL or chylomicron remnant as a ligand. Moreover, it has been recently reported that LRP takes an .alpha..sub.2 -macroglobulin/protease complex or a plasminogen activator/plasminogen activator inhibitor-1 complex as a ligand, and that LRP is a protein identical to the .alpha..sub.2 -macroglobulin receptor. When these findings are taken together, LRP is considered to have a wide variety of functions in living bodies [Herz, J., Hamann, U., Rogne, S., Myklebost, O., Gausepohl, H. and Stanley, K. K. (1989) EMBO J. 7(13), 4119-4127; Brown, M. S., Herz, J., Kowal, R. C. and Goldstein, J. L. (1991) Current Opinion in Lipidology 2, 65-72; Herz, J. (1993) Current Opinion in Lipidology 4, 107-113]. The gp 330, which was first identified as an antigen inducing rat Heymann nephritis, has been reported to have a ligand-binding capacity similar to that possessed by CRP .alpha..sub.2 -macroglobulin receptor [Raychowdhury, R., Niles, J. L., McCluskey, R. T. and Smith, J. A. (1989) Science 244, 1163-1165; Pietromonaco, S., Kerjaschki, D., Binder, S., Ullrich, R. and Farquhar, G. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 1811-1815]. In addition, recently discovered VLDL receptors, which are found to take VLDL as a ligand, are considered to have new functions including fatty acid metabolism, because they are predominantly found in tissues of the heart and muscles though they are rarely found in the liver [Takahashi, S., Kawarabayashi, Y., Nakai, T., Sakai, J. and Yamamoto, T. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 9252-9256].
Functions of these newly found receptors as lipoprotein receptors have been gradually elucidated through detailed in vitro analyses. However, significance of respective receptors in living bodies has mostly been left unknown. In addition, relations to remnant receptors, HDL receptors, etc., which have conventionally been identified or suggested by biochemical techniques, remain unknown. Presently, it is considered that these newly found receptors are products of genes different from those of the latter receptors. Thus, more lipoprotein receptors than originally guessed have become considered to participate in lipoprotein uptake into cells while interacting with each other to thereby function to maintain homeostasis of lipid metabolism in living bodies. However, from structural analyses of the genes of the aforementioned newly-identified receptors, it is predicted that the genes of these receptors that take lipoproteins as ligands are developed from the same gene from which LDL receptors was developed, and thus they are within the same genetic family. This suggests that lipoprotein receptors that have conventionally been proposed may have structures similar to those of LDL receptors.
Accordingly, an object of the present invention is to provide the gene of a novel receptor in the LDL receptor family, as well as a protein coded by the gene.
The present inventors conducted careful studies so as to attain the above object, and found that by using part of rabbit LDL receptor cDNA as a probe there can be obtained a DNA fragment coding for a peptide having a structure similar to that of LDL receptors. Moreover, when using part of the obtained cDNA as a probe, a cDNA fragment having a sequence similar to that of the cDNA can be obtained from the human tissue cDNA library. The present invention was accomplished based on these findings.